As methods of combining metallic fine particles, an electrochemical method (Y.-Y. Yu, S.-S. Chang, C.-L. Lee, C. R. C. Wang, J., Phys. Chem. B, 101, 6661 (1997) ), a chemical method (N. R. Jana, L. Gearheart, C. J. Murphy, Adv. Mater. Vol. 13, No. 18, 1389 (2001)), and a photochemical method (E Kim, J. H. Song, P. Yang, J. Am. Chem. Soc., 124, 14316 (2002)) are known in the art. In addition, as a method of producing spherical noble metal particles as coating material or coloring material of resin composition, a method of dissolving noble metal in solvent, adding a high-molecular weight compound thereto and then resolving the resultant solution is known in the art (Japanese Unexamined Patent Publication No. 11-80647). Furthermore, Japanese Unexamined Patent Publication No. 2001-64794 discloses the formation of metallic wiring patterns, using fine rods in which inorganic particulates are supported on a solid surface and absorbing Plasmon. These wiring patterns grew to a diameter of less than 100 nm and an aspect ratio of not less than 1 was obtained.
The electrochemical method is to resolve metallic ions dissolved from an anode and then grow the resolved metallic ions into rod-like fine particles by the action of a surfactant under ultrasonic irradiation. The electrochemical method requires an electrolytic apparatus, expensive conducting electrodes such as gold plates, platinum plates or silver plates, and ultrasonic irradiation, and also the obtainable amount is limited according to the size of the apparatus. Therefore, this method is unsuitable for large-scale production. Moreover, because the aspect ratio of the metal nano rods depends on the degradation of the ultrasonic transmitter or the dissolved amount of silver, the reproducibility is not uniform.
A known chemical method is to add a reducing agent to a metallic salt to initially create fine metallic species as a growing core, add a predetermined amount of solution including these metallic species to a growth solution contained in a separate vessel, and then grow rods having a rod-like structure. When the initially produced metallic species have a short working life and the growth solution contains any reducing agents, both the first reducing agent and the second reducing agent have a strong reduction ability. Hence, the growth of the metallic fine particles progresses nonuniformly and thereby metallic fine particles having low reproducibility are obtained. Further, this method produces metallic nanorods have a low concentration.
The photochemical method is to irradiate the metallic ions included in a surfactant-containing solution for long time and then produce metal nanorods. The method requires expensive apparatuses such as an ultraviolet exposure apparatus, and also the obtainable product is limited by the range of light irradiation. As a result, the yield is limited, and therefore the method is unsuitable for large-scale production.
For the production process of Japanese Unexamined Patent Publication No. 11-80647, an example employing amine as the reducing agent is disclosed. According to the disclosed method, by combining spherical noble metal particles with a high-molecular-weight pigment agent, rod-like metallic fine particles are not obtainable. Further, a high-molecular-weight dispersing agent is added to the raw material. However, the added dispersing agent is used as a protective colloid of the created noble metal particles. The dispersing agent does not grow the metallic fine particles into rods, and also no manner of employing the dispersing agent together with the reducing agent is known in the art. Furthermore, for the production process of Japanese Unexamined Patent Publication No. 2001-64794, the metallic fine particles are grown on a solid surface, and the metallic fine particles are supported on the solid surface. Therefore, the metallic fine particles can not be used separately, can not be dispersed in the various solvents, binders, and the thus obtained metallic fine particles can not be used as a composition for a coating material, etc.
In view of the above, it is an object of the present invention to address the aforementioned problems in connection with the known method of producing metallic fine particles. According to an aspect of the present invention, there is provided a process for producing the metallic fine particles by chemically reducing metallic ions, wherein the metallic fine particles have a superior reproducibility of particle size and light absorption characteristics. Further, according to another aspect of the present invention, there is provided metallic fine particles obtained by the aforementioned production process, a metallic fine particle-containing composition, and the use thereof.